This invention relates to frequency mixers and, more particularly, frequency downconverters in waveguide systems.
As is well known, when a nonlinear device intermodulates a high-frequency input signal with a local oscillator pumping signal, intermodulation products are generated at integral sums and differences of the input signal frequency and the pumping frequency. In a frequency downconverter the intermodulation product having the greatest power is the product at the intermediate frequency, defined as the intermodulation product at the frequency difference between the input signal and the pumping signal. As is known in the art, a frequency downconverter can intermodulate the input signal with a subharmonic of the conventional pumping frequency to generate the same intermediate frequency signal. The mixers of these latter types, such as that described in "Stripline Downconverter with Subharmonic Pump" by M. V. Schneider and W. W. Snell, Jr., Bell System Technical Journal, Vol. 53, No. 6, July-August 1974, pp 1179-1183, the selection of the nonlinear device or devices used to intermodulate the input and pumping signals permits the use of a pumping signal having a frequency which is a submultiple of that needed in conventional fundamentally pumped frequency downconverters. Accordingly, in a half-frequency pumped downconverter the intermediate frequency is equal to the difference between the input frequency and twice the pumping frequency.
In either a fundamentally or subharmonically pumped frequency mixer-downconverter the intermodulation product having the second highest power is at the image frequency which is symmetrical with the signal frequency with respect to the pumping frequency (in a fundamentally pumped mixer-downconverter) or twice the pumping frequency (in a half-frequency pumped mixer-downconverter). The image frequency is thus separated from the signal frequency by twice the intermediate frequency. A frequency mixer-downconverter is classified as either single sideband (SSB) or double sideband (DSB) in accordance with the impedance termination of the mixer at the image and signal frequencies. In a single sideband mixer the image frequency sees a reactive termination at the nonlinear device(s) so that all energy generated by the nonlinear device(s) is reflected back into the nonlinear device(s). In a double sideband mixer the image and signal frequencies see the same impedance match at the nonlinear device(s).
Prior art single sideband mixers have used filters at the input to the mixer to reject and filter the image frequencies. These filters, however, narrow the RF bandwidth over which the mixer can operate. In addition, the conversion loss, defined as the difference between the signal power at the input of the mixer and the intermediate frequency output power, was higher than desired. Prior art double sideband mixers have used various standard means to obtain the desired impedance matches at the input to the mixer. Prior art mixer structures have not, however, been able to be tuned for both single sideband and double sideband operation.